The interactions between viral nucleic acids and proteins leading to the formation of nucleoprotein structures, and the subsequent association of such structures with membrane proteins and lipids, are two independent, vital areas of biological investigation which merge together in physicochemical study of how murine leukemia viruses are assembled. We intend to analyze two specific aspects of this assembly problem: (1) the molecular basis for the differences between "immature", enveloped "A" -type, and "mature" murine C-type virus particles; and (2) the role played by specific proteolytic activities in viral assembly. In both aspects we will utilize ultrastructural, biochemical, and immunological approaches. High virus-producing lines of mouse cells, temperature-sensitive virus mutants, and acutely infected cells, will be used to provide viruses, cell membranes, and other needed substrates.